1. Field of the Invention
The present invention relates to an Electromagnetic Interference (EMI) low-pass filter (LPF) integrated circuit (IC), and more specifically to a LPF IC with integrated electrostatic discharge (ESD) protection and incorporating a capacitor network to enhance filter performance.
2. Description of the Prior Art
EMI interferences associated with cellular phones, radios, televisions and other electronic systems widely exist and can seriously affect normal operation of electronic products, especially in high resolution and high data rate electronics such as smart phones, laptops, LCD displays, high-definition TV, digital cameras, etc. To minimize these interferences, EMI LPF filters are widely used in many electronic products to block incoming radio frequency (RF) interferers and noises. On the other hand, electronic systems can generate and emit unwanted RF noises that will affect other electronics. It is hence critical to block any incoming and emitting EMI noises of electronics.
Using EMI filters in data lines and I/O ports is an effective way to block the EMI radiation in both directions by filtering the unwanted RF noises generated from electronic systems and/or occurring in the natural environments. In particular, wireless electronics such as smart phones and LCD are the victims of EMI effect. The data lines linking the main circuit board to the display panel are usually long and very susceptible to high frequency EMI interferers. Meanwhile, any RF noises generated from wireless devices can radiate into the environment through the data lines, which act like antennas. Therefore, the two major tasks for EMI filter are to block any high frequency RF noises and to maintain the base band signal integrity.
LPF is utilized to resolve the above EMI problems. The critical specifications for LPF EMI filter design include low pass-band insertion loss (IL), a broad pass band and high rejection-band attenuation. A low insertion loss in the low pass band ensures the desired baseband signals passing through the LPF filter with minimum signal loss. A broad pass band, defined as the frequency bandwidth from DC (e.g., 1 MHz) to the cut-off frequency (fc) measured at the −3 dB insertion loss point, allows the desired baseband signals with wider frequency spectrum (i.e., lots of useful baseband harmonic signals) to pass through the filter. Generally, a wider pass-band, i.e., a higher fc, enables higher wireless communication data rates. The baseband operation frequency is usually ⅕ to ⅓ of the fc, which guarantees good signal integrity. The rejection-band is determined by the wireless systems, typically from 800 MHz to 6 GHz. The rejection band serves to remove any high-frequency EMI interferers, which are generally associated with the carrier band frequencies in RF systems. To ensure the desired data rates and signal integrity, at least −20 dB attenuation in the rejection-band for the EMI interferes is required in EMI filter circuit designs to ensure the required signal-to-noise ratio (SNR) for the wireless systems.
FIG. 1 illustrates a conventional S-parameter measurement result for the forward transmission gain parameter (S21) of an EMI LPF showing its key specifications (i.e., specs), e.g., IL, fc, and rejection band behaviors. The insertion loss curve is always affected by parasitic parameters induced by the package and printed circuit board (PCB), causing the S21 curve bounce back in the rejection band.
Traditionally, integrated resistor-capacitor (RC) and inductor-capacitor (LC) filters are used as EMI filters. Such EMI filters usually incorporate integrated ESD protection devices to ensure system level transient voltage suppression (TVS) function. FIG. 2A and FIG. 2B show the two ideal π-type CRC and CLC EMI LPF filter circuits, respectively. FIG. 3A and FIG. 3B illustrate these two CRC and CLC EMI filter circuits with integrated ESD protection, which are Zener diodes in the examples where the anodes are connected to the input and output nodes, respectively, and the cathodes are grounded. The parasitic capacitance of the ESD diodes contribute to the two capacitors required, which requires careful design balance between the required capacitance values and the ESD diode sizes for given ESD protection level. The Zener diode ESD protection devices can be replaced by any other type of ESD protection structures in practical designs.
As a result, the basic EMI filters cannot deliver strong rejection band attenuation and very high fc to ensure excellent baseband signal integrity due to LPF filter design trade-offs. This is because, in practical LPF filter designs, to achieve the required low insertion loss and broad pass-band, while obtaining high rejection-band attenuation, are in conflict and challenging. Particularly, the S21 curve should have a very clean and high fc and a fast roll-off attenuation curve, i.e., a steep S21 roll-off curvature after the designed fc point. The conventional single-stage CRC and CLC filters shown in FIGS. 2A, 2B, 3A and 3B cannot achieve these requirements due to various IC and package parasitic effects. Alternatively, multiple-stage CRC and CLC EMI filter circuits may improve the rejection band attenuation while achieving high fc. FIG. 4A and FIG. 4B depict examples of two-stage CRC and CLC LPF circuits for this purpose, respectively. The extra frequency poles introduced by the extra capacitors and inductors in the LPF circuits can be fine-tuned to compensate frequency behaviors, hence to achieve the required fc, sharper roll-off curve and better rejection band attenuation. Similarly, higher-order multiple-stage LPF filter circuits can further improve the EMI filter performance through frequency compensation. Unfortunately, a multiple-stage LPF filter requires extra components, including resistors, capacitors or inductors, resulting in fast increase of the EMI LPF filter size, which is undesired for most small footprint electronics, such as smart phones.
On account of above, it should be obvious that there is indeed an urgent need for a new EMI circuit which can enhance the rejection band attenuation, while maintaining high fc and yet without increase of the filter size when using an EMI LPF filter.